Process and materials for aligning liquid crystals and liquid crystal displays

ABSTRACT

The present invention provides a process for preparing an optical alignment layer for aligning liquid crystals and liquid crystal displays comprising exposing polyimide layers with polarized light. The invention further describes optical alignment layers, liquid crystal displays incorporating optical alignment layers and novel polymer compositions within the class of polyimide, polyamic acids and esters thereof.

BACKGROUND OF INVENTION

The present invention relates to processes and materials for aligningliquid crystals, and liquid crystal optical elements.

Current liquid crystal display (LCD) elements include a product thatutilize a twisted nematic mode, i.e., having a structure wherein thealigning direction of nematic liquid crystal molecules is twisted by 90°between a pair of upper and lower electrode substrates, a productutilizing a supertwisted nematic mode, utilizing a birefringent effect,i.e. having a structure wherein the aligning direction of nematic liquidcrystal molecules is twisted by 180° to 300°, an in-plane-switching modewherein both electrodes controlling the liquid crystal alignment arepresent on one substrate and the direction of the liquid crystalorientation in the plane of the substrate changes upon application of anelectric field, and a product utilizing a ferroelectric liquid crystalsubstance or an antiferroelectric liquid crystal substance. Common toeach of these products is a liquid crystal layer disposed between a pairof substrates coated with a polymeric alignment layer. The polymericalignment layer controls the direction of alignment of the liquidcrystal medium in the absence of an electric field. Usually thedirection of alignment of the liquid crystal medium is established in amechanical buffing process wherein the polymer layer is buffed with acloth or other fiberous material. The liquid crystal medium contactingthe buffed surface typically aligns parallel to the mechanical buffingdirection. Alternatively, an alignment layer comprising anisotropicallyabsorbing molecules can be exposed to polarized light to align a liquidcrystal medium as disclosed in U.S. Pat. No. 5,807,498 "Process andMaterials for Aligning Liquid Crystals and Liquid Crystal OpticalElements".

The process for aligning liquid crystal media with polarized light is anon-contact method of alignment that has the potential to reduce dustand static charge buildup on alignment layers. Other advantages of theoptical alignment process include high resolution control of alignmentdirection and high quality of alignment.

Requirements of optical alignment layers for liquid crystal displaysinclude low energy threshold for alignment, transparency to visiblelight (no color), good dielectric properties and voltage holding ratios(VHR), long-term thermal and optical stability, and in many applicationsa controlled uniform pre-tilt angle.

Most liquid crystal devices, including displays, have a finite pre-tiltangle, controlled, for instance, by the mechanical buffing of selectedpolymeric alignment layers. The liquid crystal molecules in contact withsuch a layer aligns parallel to the buffing direction, but is notexactly parallel to the substrate. The liquid crystal molecules areslightly tilted from the substrate, for instance by about 2-15 degrees.For optimum performance in most display applications a finite anduniform pre-tilt angle of the liquid crystal is desirable.

Polymers used in forming optical alignment layers also must have areasonably broad processing window. Polymers used as alignment layers incommercial liquid crystal displays are generally polyimide-based systemsbecause of their good thermal and electrical properties. Thus, withinthe polyimide family, polymers also must have functionality that isstable to thermal and/or chemical imidization. In addition, polymersmust have good wetting characteristics and printability onto substratesto give uniform layers.

Several approaches have been explored to meet the performancerequirements of optical alignment layers for liquid crystal displays. Inparticular, U.S. Pat. No. 5,807,498 describes polyimide opticalalignment layers having diaryl ketones as the anisotropically absorbingmolecules. These materials can give good to excellent uniformity ofalignment of liquid crystals. However, mass production of liquid crystaldisplays generally requires materials that are more photosensitive tolight than the diaryl ketone based polyimides, and have improvedelectrical properties, especially with regard to VHR.

In further developing materials and processes for optical alignmentlayers, new classes of reactive materials have been developed thatsuggest an increased photosensitivity of polyimides by incorporatinghigher densities of anisotropically absorbing moieties. Surprisingly,the new materials exhibit improved VHR, especially at elevatedtemperatures.

SUMMARY OF INVENTION

The present invention provides a process for preparing an opticalalignment layer for aligning liquid crystals comprising: preparing apolyimide layer, comprising anisotropically absorbing molecules, on asubstrate; exposing said polyimide layer to polarized light; thepolarized light having a wavelength within the absorption band of saidanisotropically absorbing molecules; wherein the resulting exposedanisotropically absorbing molecules induce alignment of a liquid crystalmedium at an angle with respect to the major axis of the polarization ofthe incident light and along the surface of the optical alignment layer;wherein the polyimide layer comprises the structural element I whereinAr is a divalent aryl radical and A is a divalent organic radical withtwo or more ##STR1## carbons. The invention further embodies opticalalignment layers prepared by the process, liquid crystal displayelements incorporating the optical alignment layers and novel polymercompositions within the class of polyimides, polyamic acids and esters.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates the optical system used to expose the substrates toUV light.

FIG. 2 is a cross-sectional view of a LCD element of the presentinvention.

DETAILED DESCRIPTION

As used herein, the term "alignment layer" is the layer of material onthe surface of a substrate that controls the alignment of a liquidcrystal layer in the absence of an external field. A "conventionalalignment layer" herein refers to an alignment layer that will onlyalign a liquid crystal layer via processing other than optical means.For example, mechanically buffed polyimides, evaporated silicon dioxide,Langmuir-Blodgett films, have all been shown to align liquid crystals.

"Optical alignment layer" herein refers to an alignment layer thatcontains anisotropically absorbing molecules that is exposed withpolarized light sufficient to induce alignment of liquid crystals. Theoptical alignment layer can be an isotropic medium or have some degreeof anisotropy before optical alignment. Optical alignment layers may beprocessed by conventional means, such as mechanical rubbing, prior to orafter exposure to polarized light. The anisotropically absorbingmolecules of the optical alignment layers exhibit absorption propertieswith different values when measured along axes in different directions.The anisotropic absorbing molecules exhibit absorption bands between 150nm and about 2000 nm. Most preferable optical alignment layers for thepresent invention have absorbance maxima of about from 150 to 400 nm andespecially about from 300 to 400 nm.

Polymers especially useful and preferred as optical alignment layers arepolyimides. Polyimides are known for their excellent thermal andelectrical stability properties and these properties are useful inoptical alignment layers for liquid crystal displays. The preparation ofpolyimides is described in "Polyimides", D. Wilson, H. D. Stenzenberger,and P. M. Hergenrother Eds., Chapman and Hall, New York (1990).Typically polyimides are prepared by the condensation of one equivalentof a diamine with one equivalent of a dianhydride in a polar solvent togive a poly(amic acid) prepolymer intermediate. Copolymer polyimides areprepared by the condensation of one or more diamines with one or moredianhydrides to give a copolyamic acid.

An alternative intermediate to polyimides are poly(amic esters) that canbe made by esterification of poly(amic acids) with alcohols. Thepoly(amic esters) undergo thermal imidization to form polyimides.

Thus, poly(amic acids) and poly(amic esters) are considered to beclosely related percursors to polyimides of the invention. Therefore,they are considered further embodiments of this invention. Furthermore,preimidized polyimides derived from chemical or thermal imidzation ofpoly(amide acids) or poly(amide esters) are also considered anembodiment of the invention.

The process of the invention requires a polyimide comprising thestructural element I ##STR2## wherein Ar is a divalent aryl radical andA is a divalent organic radical with two or more carbons.

By "divalent aryl radical" is meant that the radical may be comprised ofaromatic and heteroaromatic rings containing one to six rings. Theradical may have fused rings to form a polycyclic radical. The radicalmay have rings covalently linked through a covalent bond or a linkinggroup. The radical may have a mix of fused and covalently linked rings.Preferred polyimides for the process are those in which Ar is selectedfrom the group of ##STR3## wherein X₁, independently, is selected fromthe group of H, Cl, Br, F, --CN, --CF₃, --(R)₂ N--, --OR and R, Z₁ isselected from the group of covalent bond, --O--, --NR₁ --, --(R₁)₂ C--,--CH₂ CH₂ --, and --C(O)--, Z₂ is selected from the group --O--, --NR₁--, and --(R₁)₂ C--, R₁ is H or lower alkyl group and R is a lower alkylgroup.

Another embodiment of the invention are novel polymers within the classof polyimides, polyamic acids and esters thereof, characterized in thatthey comprise identical or different repeat units selected from one ormore of the formula ##STR4## wherein B is hydrogen or a monovalentorganic group derived from an alcohol after formal removal of thehydroxyl group, X₂ is an electron withdrawing group having a positive σ,Ar is a divalent aryl group and the carboxyl groups are in an orthoposition relative to each other. The propensity for an organicsubstituent to donate or withdraw electron density from a electronicsystem is described by the Hammett equation. J. March describes theHammett equation in detail in "Advanced Organic Chemistry, Reactions,Mechanism, and Structure", McGraw-Hill, Publishers, New York 1977, p.252-255. A positive value of a indicates an electron-withdrawing groupand a negative value an electron-donating group. More preferred polymerswithin this class are those wherein X₂ is selected from the group of--CN, --CF₃, CO₂ R, F, Cl, Br, and --NO₂ wherein R is a lower alkylgroup. Most preferred polymers within this class are those wherein X₂ isselected from the group of --CN and --CF₃ and Ar is selected from thegroup described above.

The dianhydrides required for the synthesis of these polyimides areavailable by synthesis and are referred to asbis-(dicarboxyphenylketone)aryl dianhydrides. Severalbis(3,4-dicarboxyphenylketone)aryl dianhydrides are listed in Table 1.J. R. Pratt, et al. (Polymer Engineering and Science, 1989, 29, 63-68)describes the synthesis of dianhydride 1. French Patent SpecificationNo. 1,601,094 describes the synthesis of dianhydride 2. Dianhydrides 3-9are described by Sonnenberg in U.S. Pat. No. 4,002,645.

A wide variety of other dianhydrides, of course, may be used in formingcopolyamic acids. Preferred are 3,3',4,4'-benzophenonetetracarboxylicdianhydride (BTDA), 2,2'-dichloro-4,4',5,5'-benzophenone tetracarboxylicdianhydride and the polycyclic diaryl ketone dianhydrides described byPfeifer, et al., in U.S. Pat. No. 4,698,295 and hereby incorporated byreference. Specific examples of other tetracarboxylic dianhydridecomponents include aromatic dianhydrides such as pyromelliticdianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride,1,2,5,6-naphthalenetetracarboxylic dianhydride,1,4,5,8-naphthalenetetracarboxylic dianhydride,3,3'4,4'-biphenyltetracarboxylic dianhydride,2,3,2',3'-biphenyltetracarboxylic dianhydride,bis(3,4-dicarboxyphenyl)ether dianhydride,bis(3,4-dicarboxyphenyl)diphenylsulfone dianhydride,bis(3,4-dicarboxyphenyl)methane dianhydride,2,2-bis(3,4-dicarboxyphenyl)propane dianhydride,1,1,1,3,3,3-hexafluoro-2,2-bis(3,4-dicarboxyphenyl)propane dianhydride,bis(3,4-dicarboxyphenyl)dimethylsilane dianhydride,2,3,4,5-pyridinetetracarboxylic dianhydride; alicyclic tetracarboxylicdianhydrides such as 1,2,3,4-butanetetracarboxylic dianhydride,1,2,3,4-cyclobutanetetracarboxylic dianhydride,1,2,3,4-cyclopentanetetracarboxylic dianhydride,1,2,4,5-cyclohexanetetracarboxylic dianhydride,2,3,5-tricarboxycyclopentylacetic acid dianhydride and3,4-dicarboxy-1,2,3,4-tetrahydro-1-naphthalenesuccinic dianhydride; andtheir acid and acid chloride derivatives.

A variety of diamines may be useful in the preparation of the polyimidesand copolyimides useful in the invention including aromatic diaminessuch as are 2,5-diaminobenzonitrile,2-(trifluoromethyl)-1,4-benzenediamine, p-phenylenediamine,2-chloro-1,4-benzenediamine, 2-fluoro-1,4-benzenediamine,m-phenylenediamine, 2,5-diaminotoluene, 2,6-diaminotoluene,4,4'-diaminobiphenyl, 3,3'-dimethyl-4,4'-diaminobiphenyl,3,3'-dimethoxy-4,4'-diaminobiphenyl, diaminodiphenylmethane,diaminodiphenyl ether, 2,2-diaminodiphenylpropane,bis(3,5-diethyl-4-aminophenyl)methane, diaminodiphenylsulfone,diaminonaphthalene, 1,4-bis(4-aminophenoxy)benzene,4,4'-diaminobenzophenone, 3,4'-diaminobenzophenone,1,4-bis(4-aminophenyl)benzene, 9,10-bis(4-aminophenyl)anthracene,1,3-bis(4-aminophenoxy)benzene, 4,4'-bis(4-aminophenoxy)diphenylsulfone,2,2-bis[4-(4-aminophenoxy)phenyl]propane,2,2-bis(4-aminophenyl)hexafluoropropane and2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane; alicyclic diaminessuch as bis(4-aminocyclohexyl)methane; and aliphatic diamines such astetramethylenediarnine and hexamethylene diamine. Further,diaminosiloxanes such as bis(3-aminopropyl)tetramethyldisiloxane may beused. Such diamines may be used alone or in combination as a mixture oftwo or more of them. Preferred diamines for preparing copolyimides are2,5-diaminobenzonitrile, 2-(trifluoromethyl)-1,4-benzenediamine,2-(N,N-diallylamino)1,4-benzenediamine,1-(N,N-diallylamino)-2,4-benzenediamine and 1,4-phenylene diamine. Morepreferred diamines are 2,5-diaminobenzonitrile,2-(trifluoromethyl)-1,4-benzenediamine and2-(N,N-diallylamino)1,4-benzenediamine. Table 2 lists examples ofpreferred amines, including diamines and monoamines.

                  TABLE 1                                                         ______________________________________                                        Bis(3,4-dicarboxyphenylketone)aryl dianydrides useful in                      preparing polyimides for optical alignment layers.                             ##STR5##                                                                     No.      --Ar-- Structure                                                     ______________________________________                                                  ##STR6##                                                            2                                                                                       ##STR7##                                                            3                                                                                       ##STR8##                                                            4                                                                                       ##STR9##                                                            5                                                                                       ##STR10##                                                           6                                                                                       ##STR11##                                                           7                                                                                       ##STR12##                                                           8                                                                                       ##STR13##                                                           9                                                                                       ##STR14##                                                           ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Aromatic amines including diamines and monoamines useful in                   preparing preferred polyimides for optical alignment layers.                  No.  Structure                                                                ______________________________________                                              ##STR15##                                                               2                                                                                   ##STR16##                                                               3                                                                                   ##STR17##                                                               4                                                                                   ##STR18##                                                               5                                                                                   ##STR19##                                                               6                                                                                   ##STR20##                                                               7                                                                                   ##STR21##                                                               ______________________________________                                    

                                      TABLE 3                                     __________________________________________________________________________    Optical Alignment of Polyimide Compositions, Processing Parameters and        Results.                                                                                        Exposure                                                    Example                                                                            Polyimide Composition                                                                      Scan speed                                                                           Alignment                                                                          VHR                                             No.  Dianhydride No.:Diamine No.                                                                mm/sec quality                                                                            RT 75° C.                                __________________________________________________________________________    1    1:1          0.75   Δ+                                                                           0.892                                                                            0.463                                        "    "            1.5    Δ                                                                            0.917                                                                            0.530                                        2    1:2          0.75   Δ+                                                                           0.944                                                                            0.612                                        "    "            1.5    ◯                                                                      0.964                                                                            0.689                                        3    1:DAABD + 5  0.75    Δ++                                                                         0.872                                                                            0.596                                        "    "            1.5    Δ+                                                                           0.904                                                                            0.613                                        4    BTDA:1       0.75   ◯                                                                      0.926                                                                            0.548                                        "    "            1.5    ◯                                                                      0.922                                                                            0.508                                        5    BTDA:2       1.5     Δ++                                                                         0.933                                                                            0.525                                        6    BTDA:DAABD + 5                                                                             0.75   ◯+                                                                     0.855                                                                            0.387                                        "    "            1.5    ◯+                                                                     0.909                                                                            0.490                                        __________________________________________________________________________     ◯ Good Alignment, low flow effects, uniform.                      Δ Fair alignment, flow effects, some nonuniformity (mottled or          cloudy background)                                                            X Poor alignment, severe flow effects, nonuniform.                            + Levels of improvement, Δ < Δ+ < Δ++ < ◯- 

Other preferred diamines are the pre-tilt inducing diamines described inU.S. Pat. No. 5,817,743, and pending U.S. application Ser. No.08/859,404 titled "Polarizable Amines and Polyimides for OpticalAlignment of Liquid Crystals", now U.S. Pat. No. 6,0840,057, and Ser.No. 09/425,589 titled "Materials for Inducing Alignment in LiquidCrystals and Liquid Crystal Displays," hereby incorporated by reference.A specific pre-tilt inducing diamine used in the examples is diamine 5listed in Table 2.

Preferably the novel polyimides and copolyimides of the inventioncomprise 5 to 100 mol %, and more preferably 20 to 90 mol %, of apre-tilt inducing diamine.

In preparing polyamic acids for optical alignment layers the molar ratioof diamine to dianhydride usually is 1:1, but can vary between 0.8:1 to1.2:1. The preferred ratio of diamine to dianhydride is between 0.9:1and 1.1:1.

To prepare the optical alignment layers the poly(amic acid) solutions orpreimidized polyimide solutions are coated onto desired substrates.Coating is usually accomplished with 2 to 30 wt % solids. Anyconventional method may be used to coat the substrates includingbrushing, spraying, spin-casting, meniscus coating, dipping or printing.The preferred techniques for coating substrates are spinning andprinting. However, the optical alignment materials of the invention arenot limited to use in printing or spinning processes.

The coated substrates are heated in an oven under an inert atmosphere,for instance nitrogen or argon, at elevated temperatures usually notexceeding 300° C. and preferably at or below 180° C. for about from 1 to12 hours, preferably for about 2 hours or less. The heating processremoves the solvent carrier and may be used to further cure the polymer.For instance, the poly(amic) acid films are thermally cured to generatepolyimide films.

The concentration of polymer and choice of solvents can affect theoptical alignment quality, pretilt and voltage holding ratio (VHR). Forexample, the optical alignment quality has been observed to improveunder the same exposure conditions when the concentration of polymer isdecreased in solution. The choice of solvent and/or co-solvents can alsoaffect the alignment quality. A correlation between film thickness andalignment quality also is evident. In particular, the optical alignmentquality improves with decreasing thickness. Similarly, VHR increaseswith decreasing film thickness.

The optical alignment layers are exposed to polarized light to inducealignment of liquid crystals. By "polarized light" is meant light thatis elliptically and /or partially polarized such that the light is morepolarized along one axis (referred to as the major axis) versus theorthogonal axis (referred to as the minor axis). In this invention thepolarized light has one or more wavelengths of about from 150 to 2000 nmand preferably of about from 150 and 1600 nm and more preferably aboutfrom 150 to 800 nm. Most preferably, the polarized light has one or morewavelengths of about from 150 to 400 nm, and especially about from 300to 400 nm. A preferred source of light is a laser, e.g., an argon,helium neon, or helium cadmium. Other preferred sources of light aremercury arc deuterium and quartz tungsten halogen lamps, xenon lamps,microwave excited lamps and black lights in combination with apolarizer. Polarizers useful in generating polarized light fromnonpolarized light sources are interference polarizers made fromdielectric stacks, absorptive polarizers, diffraction gratings andreflective polarizers based on Brewster reflection. With lower powerlasers or when aligning small alignment regions, it may be necessary tofocus the light beam onto the optical alignment layer.

By "exposing" is meant that polarized light is applied to the entireoptical alignment layer or to a portion thereof. The light beam may bestationary or rotated. Exposures can be in one step, in bursts, inscanning mode or by other methods. Exposure times vary widely with thematerials used, etc., and can range from less than 1 msec to over anhour. Exposure may be conducted before or after contacting the opticalalignment layer with the liquid crystal medium. Exposing can beaccomplished by polarized light transmitted through at least one maskhaving a pattern or with a beam of polarized light scanned in a pattern.Exposing may be accomplished using interference of coherent opticalbeams forming patterns, i.e., alternating dark and bright lines.

Exposing also can consist of two or more exposure steps wherein theconditions of each step such as angle of incidence, polarization state,energy density, and wavelength are changed. At least one of the stepsmust consist of exposure with linearly polarized light. Exposures canalso be localized to regions much smaller than the substrate size tosizes comparable to the entire substrate size. A preferred method ofdual exposing comprises a two step process of:

(a) exposing at least one optical alignment layer to polarized light ata normal incidence, and

(b) exposing the optical alignment layer to polarized light at anoblique incidence. In this preferred process the oblique incidenceexposure helps predominately define the pre-tilt angle of the liquidcrystal when placed in contact with the optical alignment layer.

Exposure energy requirements vary with the formulation and processing ofthe optical alignment layer prior and during exposure. A preferred rangeof exposure energy is about from 0.001 to 100 J/cm² and most preferredrange of exposure energy is about from 0.001 to 5 J/cm². Lower exposureenergy is most useful in large scale manufacturing of optical alignmentlayers and liquid crystal display elements. Lower exposure energy alsominimizes the risk of damage to other materials on the substrates.

The quality of alignment and electrical properties of the liquid crystalcell assembled from exposed substrates can be improved by heating thesubstrates after exposure but prior to assembly and/or filling of thecell. This additional heating of the substrates and/or cells is not arequirement of the process but may give beneficial results.

Applying a liquid crystal medium to the optical alignment can beaccomplished by capillary filling of a cell, by casting of a liquidcrystal medium onto an optical alignment layer, by laminating apreformed liquid crystal film onto an optical alignment layer or byother methods. Preferred methods are capillary filling of a cell,injection filling and casting of a liquid crystal medium onto an opticalalignment layer. Optical alignment layers are pre-exposed to polarizedlight or they are exposed after contacting the liquid crystal medium.

A cell can be prepared by using two coated substrates to provide asandwiched layer of liquid crystal medium. The pair of substrates canboth contain optical alignment layers or a conventional alignment layer(e.g., mechanically buffed) can be used as the second alignment layercomprising the same or a different polymer.

As liquid crystal substances used for liquid crystal optical elements,nematic liquid crystal substances, ferroelectric liquid crystalsubstances, vertical alignment liquid crystals (negative dielectricliquid crystals) etc. are usable. Useful liquid crystals for theinvention described herein include positive dielectric liquid crystalsincluding 4-cyano-4'-alkylbiphenyls, 4-cyano-4'-alkyloxybiphenyls,4-alkyl-(4'-cyanophenyl)cyclohexanes,4-alkyl-(4'cyanobiphenyl)cyclohexanes, 4-cyanophenyl-4'-alkylbenzoates,4-cyanophenyl-4'alkyloxybenzoates, 4-alkyloxyphenyl-4'-cyanobenzoates,4-alkylphenyl-4'alkylbenzoates, 1-(4'-alkylphenyl)-4-cyanopyrimidines,1-(4'-alkyloxyphenyl)-4-cyanopyrimidines and1-(4-cyanophenyl)-4-alkylpyrimidines. Other useful liquid crystals arenew superfluorinated liquid crystals available from EM Industries,(Hawthrone NY) including the commercial materials: ZLI-5079, ZLI-5080,ZLI-5081, ZLI-5092, ZLI-4792, ZLI-1828, MLC-2016, MLC-2019, MLC-6252 andMLC-6043. Other useful nematic materials for practicing the inventioninclude the commercial liquid crystals available from Dinippon Ink andChemicals, Inc. (Tokyo, Japan) including the DLC series: 22111, 22112,22121, 22122, 23070, 23170, 23080, 23180, 42111, 42112, 42122,43001,43002, 43003, 63001, 63002, 63003, 63004, and 63005.

Polymerizable liquid crystal monomers also are useful in the invention.Preferred are those disclosed in U.S. Pat. No. 5,846,452, herebyincorporated by reference.

The invention is not limited to the use of liquid crystals definedabove. One skilled in the art will recognize that the invention will beof value with many diverse liquid crystal structures and formulationscontaining mixtures of liquid crystals.

The exposed optical alignment layer induces alignment of a liquidcrystal medium at an angle with respect to the major axis of thepolarization of the incident light beam and along the surface of theoptical alignment layer. One skilled in the art will recognize that theprocess allows control of the alignment of a liquid crystal medium inany desired direction within the plane of the optical alignment layer bycontrolling the conditions of the polarized light exposure.

A liquid crystal display element of the invention is composed of anelectrode substrate having at least one side-chain polyimide opticalalignment layer, a voltage-impressing means and a liquid crystalmaterial. FIG. 2 illustrates a typical liquid crystal display element,comprising a transparent electrode 13 of ITO (indium-tin oxide) or tinoxide on a substrate 12 and optical alignment layers 14 formed thereon.The optical alignment layers are exposed to polarized light of awavelength or wavelengths within the absorption band of theanisotropically absorbing molecules. A spacer concurrently with asealing resin 15 is intervened between a pair of optical alignmentlayers 14. A liquid crystal 16 is applied by capillary filling of thecell and the cell is sealed to construct a liquid crystal displayelement. Substrate 12 may comprise an overcoat film such as aninsulating film, a color filter, a color filter overcoat, a laminatedpolarizing film etc. These coatings and films are all considered part ofthe substrate 12. Further, active elements such as thin filmtransistors, a nonlinear resistant element, etc. may also be formed onthe substrate 12. These electrodes, undercoats, overcoats, etc. areconventional constituents for liquid crystal display elements and areusable in the display elements of this invention. Using the thus formedelectrode substrate, a liquid crystal display cell is prepared, and aliquid crystal substance is filled in the space of the cell, to preparea liquid crystal display element in combination with avoltage-impressing means.

Optical alignment layers of the invention are compatible with all liquidcrystal display modes. A liquid crystal display element of the inventioncan comprise a variety of display configurations including twistednematic, super twisted nematic, in-plane-switching, vertical alignment,active-matrix, cholesteric, polymer dispersed, ferroelectric,anti-ferroelectric and multi-domain liquid crystal displays. Althoughthe display modes demonstrated in this specification are primarilytwisted nematic, the optical alignment layers of the invention are notlimited to use in twisted nematic liquid crystal displays.

Optical alignment layers of the invention are useful in many otherliquid crystal devices other than liquid crystal displays. These includeelectro-optical light modulators, all-optical light modulators, erasableread/write optical data storage media; diffractive optical componentssuch as gratings, beamsplitters, lenses (e.g., Fresnel lenses), passiveimaging systems, Fourier processors, optical disc and radiationcollimators; binary optical devices formed by combining refractive anddiffractive optics including eyeglasses, cameras, night vision goggles,robotic vision and three-dimensional image viewing devices; andholographic devices such as heads-up displays and optical scanners.

Voltage Holding Ratio (VHR) is a critical electrical parameter forliquid crystal displays. VHR is a measure of the LCDs ability to retaina voltage during the time between pixel updates (frame time). The typeof liquid crystal, alignment layers and cell geometry can all affect themeasured VHR value. In the examples to follow, liquid crystal test cellscomprising soda-lime substrates with patterned indium-tin-oxide (ITO)transparent electrodes are described. The overlap of the electrodes wasabout 1 cm² after the test cell was assembled. Approximately 2-3 inchwire leads were attached to the patterned ITO electrodes using anultrasonic solder iron after the test cell is assembled but prior tofiling. The leads were attached to a VHR measurement system (ElsiconVHR-100 Voltage Holding Ratio Measurement System, Wilmington, Delaware)using test clips after the cell was filled and annealed. The VHR for theexamples was measured for a 20 msec frame time, which is typically usedfor measuring VHR.

The performance characteristics of liquid crystal display test cells inExamples 1-3 and comparative examples 4-6 are summarized in Table 3.Comparison of data presented in Table 3 reveals that, in general, higherexposure energy (lower scan speed) for the materials leads to lower VHRat room temperature (RT) and elevated temperature (75° C.). However,comparison of examples 3 and 6, containing pre-tilt inducing diamines,indicates that VHR's at 75° C. are significantly higher in example 3,comprising a bis(3,4-dicarboxyphenylketone)aryl dianhydride, than inexample 6, comprising BTDA. Also, there is only a small decrease in VHRat higher exposure energy in example 3, whereas with BTDA there isgreater than a 20% drop in VHR at 75° C. Furthermore, example 2 showsthat alignment quality improves as exposure energy is decreased, and acomparison of example 2 with 5 indicate a significantly improved VHR atelevated temperature in example 2. Thus, it is clear that thebis(dicarboxyphenylketone)aryl dianhydrides exhibit a positive influenceon VHR performance of polyimides at elevated temperature.

Some Examples use 1-(N,N-diallylamino)-2,4-benzenediamine (DAABD) as acomonomer. This material was prepared in the following manner:

A mixture of 2,4-dinitrofluorobenzene (9.3 g), N-methylpyrrolidinone(NMP, 50 mL), diallylamine (5.82 g) and potassium carbonate (6.9 g) wasstirred at ambient temperature for 1 h. The mixture was poured intowater and extracted with ethyl ether. The extract was washed twice withwater, once with saturated brine solution, and dried over magnesiumsulfate. Concentration of the extract gaveN,N-diallyl-2,4-dinitrobenzenamine as a yellow oil (14.6 g).

The above yellow oil (14.6 g) was treated with a solution of tin (II)chloride dihydrate (90.0 g,), 10 N hydrochloric acid (75 mL) and ethanol(250 mL) at 55-60° C. for 14.5 h. The mixture was poured into ice waterand basified with cold 20 wt % potassium hydroxide solution (750 g). Themixture was extracted with ethyl ether, the extracts washed with waterthree times, washed with saturated brine solution, and dried (MgSO₄).The mixture was concentrated, purified by chromatography followed byKugelrohr distillation (115-120° C., 0.1 mm Hg) to give1-(N,N-diallylamino)-2,4-benzenediamine as yellow oil. ¹ H NMR (CDCl₃)6.79 (d, 1H), 6.08 (m, 2H), 5.80 (m, 2H), 5.10 (m, 4H), 3.45 (dt, 4H),4.0 (bs) and 3.5 (bs).

The following procedures describes the synthesis of diamine 5, used inexamples:

A mixture of 1H,1H-perfluorooctanol (30.0 g, 0.075 mol),1,4-dibromo-2-butene (48.0 g, 0.25 mol), Aliquat 336 (1.5 g), toluene(150 mL) and potassium hydroxide (0.075 mol, 4.95 g) in water (50 mL)was heated to 80-90° C. for 3 hr. The mixture was extracted withwater-ethyl ether. The extract was washed with water two times, washedwith saturated sodium chloride solution, dried (MgSO₄) and concentratedto an oil. Excess dibromide was removed by recrystallization in hexane.The remaining oil was Kugelrohr distilled (0.1 mm Hg, 70-95° C.) to give28.0 g of the 1-bromo-4-(1H, 1H,-perfluorooctyloxy)-2-butene.

The 1-bromo-4-(1H,1H,-perfluorooctyloxy)-2-butene (28.0 g) was added toa mixture of 40 wt % methyl amine (59 mL), tetrahydrofuran (60 mL) andethanol (40 mL) at 40° C. and the mixture stirred at ambient temperaturefor 2 hr. The mixture was basified with 20 wt % potassium hydroxide(KOH, 15 g), extracted, concentrated and distilled to give 18.0 g1-(N-methylamino)-4-(1H,1H,-perfluorooctyloxy)-2-butene (0.1 mmHg,85-110° C.).

A mixture of 1-(N-methylamino)-4-(1H, 1H,-perfluorooctyloxy)-2-butene(17.3 g), 3-fluoro-4-nitroaniline (5.46 g), triethylamine (7 mL) and NMP(80 mL) was heated 16 hr at 80-90° C. The mixture was extracted in thenormal fashion and purified by chromatography to give 15.6 g of the3-substituted nitroamine.

The nitroamine (15.2 g, 25 mmol) was treated with tin (II) chloridedihydrate (24.6 g, 0.11 mol), 10 N hydrochloric acid (20 mL) and ethanol(200 mL) for 16 hr at 40-45° C. The mixture was diluted with cold water,basified with 20 wt % KOH (220 g), and extracted in the normal fashion.Purification by chromatography on silica gel and crystallization gavediamine 5 (mp 46-47° C.).

The following Examples are meant to exemplify the embodiments and arenot meant to limit the scope of the invention. Dianhydrides and diaminesused in the formulations are identified by the numbers in Tables 1 and2.

EXAMPLE 1

A mixture of dianhydride 1 (213 mg, 0.50 mmol),2-(trifluoromethyl)-1,4-benzenediamine (diamine 1, 88 mg, 0.50 mmol) andγ-butyrolactone (1.20 g) was stirred at 18° C. for 18 h under a nitrogenatmosphere. The polyamic acid solution was diluted to 5 wt % withγ-butyrolactone (4.5 g).

Two 0.9 inch by 1.2 inch by 1 millimeter thick soda lime glasssubstrates with transparent indium-tin-oxide (ITO) coatings (DCI, Inc.Lenexa, Kans. 66219) were spin-coated and cured with the polyamic acidformulation to give optical alignment layers. Spin coating was achievedby filtering the prepolymer solution through a 0.45 micron Teflon filtermembrane onto the surface of clean ITO substrates. The substrates werespun at 2500 RPM for 1 minute to produce uniform thin films. Theresultant thin films were cured under nitrogen for 0.25 hr at 80° C.followed by 1 h at 180° C.

FIG. 1 is a schematic of the experimental set-up used to expose thesubstrates. The laser beam of about 1 cm diameter from laser 1,polarized along direction 2, entered a polarizing rotator and beamsplitter combination 3 and, upon exiting, two polarization components 6and 7 separated as they propagated away from 3. The wavelength range ofthe laser was 300-336 nm. By adjusting the polarizing rotator in 3, theratio of optical power in 6 and 7 can be adjusted and, in this case, theratio was adjusted to be 1:6. The total power in 6 and 7 was about 500mW. Mirrors 5 reflected 6 and 7 through cylindrical lenses 8 and 9 withfocal lengths of 5 cm and 10 cm, respectively. After passing throughcylindrical lenses 8 and 9, 6 and 7 were focused into lines of about 1cm×0.2 cm onto the substrate(s) 10. The separation between the twoparallel focused lines was about 1.5 mm. As depicted in FIG. 1, thesubstrates 10 were scanned perpendicular to the focused lines. Since thefocused line lengths of about 1 cm was smaller than the desired exposurearea, after scanning one time, the substrates were stepped 1.5 mmperpendicular to the scan direction (along the focused lines). The stepand scan 11 were repeated until the entire substrate area was exposed.The scan speed for this exposure was 0.75 mm/s.

After exposure, the substrates were assembled with orthogonalorientation of the optically generated alignment direction. The cellthickness was about 4 microns. The cell was subsequently capillaryfilled with nematic liquid crystals suitable for active matrix liquidcrystal displays. As expected, the liquid crystals were observed toalign in a twisted nematic orientation when viewed between polarizers.Upon annealing the liquid crystal cell above the liquid crystalisotropic point (120° C. for 30 minutes), the uniformity of thealignment was observed to improve and was of fair quality.

A further trial using a scan speed of 1.5 mm/sec resulted in similaralignment quality as the 0.75 mm/s scan speed after anneal.

EXAMPLE 2

A mixture of dianhydride 1 (213 mg, 0.50 mmol), 2,5-diaminobenzonitrile(diamine 2, 66.6 mg, 0.50 mmol) and γ-butyrolactone (1.12 g) was stirredat 18° C. for 18 h under a nitrogen atmosphere. The polyamic acidsolution was diluted to 5 wt % with γ-butyrolactone (4.2 g). Furtherprocessing to prepare optical alignment layers and display test cellswas accomplished as described in Example 1.

EXAMPLE 3

A mixture of dianhydride 1 (142 mg, 0.333 mmol),1-(N,N-diallylamino)-2,4-benzenediamine (DAABD, 6.8 mg, 0.033 mmol),diamine 5 (176.8 mg, 0.300 mmol) and NMP (1.30 g) was stirred at 18° C.for 18 h under a nitrogen atmosphere. The polyamic acid solution wasdiluted to 5 wt % with γ-butyrolactone (3.09 g) and NMP (1.79 g).Further processing to prepare optical alignment layers and display testcells was accomplished as described in Example 1. At a scan speeds of0.75 and 1.5 mm/sec the pretilt was measured with the PAS-301 PretiltAnalysis System (Elsicon, Inc, Wilmington, Del.) to be 0.18 and 1.8degrees, respectively. No reverse tilt disclinations were observed uponswitching the cells.

EXAMPLE 4 (COMPARATIVE)

A mixture of 3,3',4,4'-benzophenonetetracarboxylic dianhydride (6.44 g),2-(trifluoromethyl)-1,4-benzenediamine (diamine 1, 3.52 g) andγ-butyrolactone (40 g) was stirred at room temperature for 24 h under anitrogen atmosphere. The solution was diluted to a 10 wt % solution withγ-butyrolactone (49.7 g) and filtered through a 0.45 micron Teflonmembrane filter. The solution was diluted to 3.5 wt % solution and spincoated, cured and exposed to polarized light as described in Example 1.

EXAMPLE 5 (COMPARATIVE)

A mixture of 3,3',4,4'-benzophenonetetracarboxylic dianhydride (6.44 g),2,5-diaminobenzonitrile (diamine 2, 2.66 g) and γ-butyrolactone (37.8 g)was stirred at room temperature for 20 h under a nitrogen atmosphere.The solution was diluted to a 10 wt % solution with γ-butyrolactone(43.9 g) and filtered through a 0.45 micron Teflon membrane filter. Thesolution was diluted to 3 wt % solution and spin coated, cured andexposed to polarized light as described in Example 1.

EXAMPLE 6 (COMPARATIVE)

A mixture of 3,3',4,4'-benzophenonetetracarboxylic dianhydride (3.69 g,11.47 mmol), 1-(N,N-diallylamino)-2,4-benzenediamine (0.233 g, 1.15mmol), diamine 5 (6.08 g, 10.32 mmol) and NMP (39.6 g) was stirred at18° C. for 18 h under a nitrogen atmosphere. The polyamic acid solutionwas diluted to 10 wt % with γ-butyrolactone (45.1 g) and NMP (5.03 g).For spinning, the polyamic acid solution was further diluted to 5 wt %with 50/50 γ-butyrolactone/NMP solution. At a scan speed of 0.75 and 1.5mm/sec the pretilt was measured to be 21.9 and 29 degrees, respectively.No reverse tilt disclinations were observed upon switching the cells.

What is claimed is:
 1. A process for preparing an optical alignmentlayer for aligning liquid crystals comprising:preparing a polyimidelayer, comprising anisotropically absorbing molecules, on a substrate,exposing said polyimide layer to polarized light; the polarized lighthaving a wavelength within the absorption band of said anisotropicallyabsorbing molecules; wherein the resulting exposed anisotropicallyabsorbing molecules induce alignment of a liquid crystal medium at anangle with respect to the major axis of the polarization of the incidentlight and along the surface of the optical alignment layer;wherein thepolyimide layer comprises the structural element I ##STR22## wherein Aris a divalent aryl radical and A is a divalent organic radical with twoor more carbons.
 2. A process of claim 1 wherein Ar is selected from thegroup of ##STR23## wherein X₁, independently, is selected from the groupof H, Cl, Br, F, --CN, --CF₃, --(R)₂ N--, --OR and R, Z₁ is selectedfrom the group of covalent bond, --O--, --NR₁ --, --(R₁)₂ C--, --CH₂ CH₂--, and --C(O)--, Z₂ is selected from the group --O--, --NR₁ --, and--(R₁)₂ C--, R₁ is H or lower alkyl group and R is a lower alkyl group.3. An optical alignment layer prepared by the process of claim
 1. 4. Aliquid crystal display element comprising at least one optical alignmentlayer of claim 3.